Could Titan's seas harbor life?

This infrared image from the Cassini orbiter shows the hydrocarbon lake known as Kraken Mare toward the northern edge of the disk. The dark Senkyo sand sea dominates the central area of the image.

A fresh photo from the Cassini orbiter shows the hydrocarbon-rich seas and dunes of Titan, a Saturnian moon that might be capable of sustaining life as we don't know it.

The picture, published today on the websites of NASA's Saturn mission and Cassini's imaging team, shows the huge sea known as Kraken Mare as a dark spot on the northern edge of Titan's disk. The dark Senkyo dune field is front and center. Cassini's narrow-angle camera captured the view in near-infrared wavelengths from a distance of 1.2 million miles (1.9 million kilometers) on Sept. 14.

Titan is totally shrouded in smog, but Cassini's camera filters are set up to pierce through the haze and spot details on the surface below. The cold condtions on the moon are such that hydrocarbons such as ethane and methane can exist in liquid form. This rare picture from Cassini shows the glint of sunlight off the sheen of Kraken Mare, which is larger than the Caspian Sea on Earth. (And yes, Kraken is named after the mythical sea creature. "Mare" is Latin for "sea.")

NASA / JPL

This image, obtained using Cassini's Visual and Infrared Mapping Spectrometer, shows the first observed flash of sunlight reflected off a lake on Saturn's Titan moon.

Titan's seas, lakes and rivers of hydrocarbons are among the reasons why the murky Saturnian moon ranks higher than Mars on a recently published list assessing planetary habitability. That may sound strange, considering that the typical temperature on Titan is 289 degrees below zero Fahrenheit (-178 degrees Celsius). But Dirk Schulze-Makuch, an astrobiologist at Washington State University who helped put together the list, told me that it makes sense to rank Titan as the top prospect for extraterrestrial habitability.

"If you think about it, Titan has a thick protective atmosphere like Earth's, similar to the early Earth atmosphere," he said. "It has a lot of nitrogen and methane in it, and Titan has hydrocarbon lakes, energy sources. There's a lot of possibility on Titan — if you objectively evaluate the possibility of life on Titan, I would agree."

He cautioned, however, that life on Titan may not take the form of life on Earth. Titanian life would have to thrive on methane rather than oxygen or carbon dioxide. Last year, some researchers were wound up by reports that hydrogen was flowing down through the moon's atmosphere and disappearing at the surface, and that acetylene was less abundant than expected. That could be consistent with the behavior of methane-based life forms. There are other possible explanations, however. It'll be another decade at least before another probe can go to Titan to sort out the truth.

Schulze-Makuch cautioned that comparing Titan with Mars and Earth "is a little like comparing apples and oranges."

"Current Titan seems to be more favorable to life than current Mars, but it's 'life as we don't know it,'" he said. "It would have to be different. For Mars, though, the thing is, early Mars and current Mars are very different. Early Mars was more favorable to life. Early Mars comes out better than Titan."

That's the main point of NASA's $2.5 billion Mars Science Laboratory mission, which was launched over the weekend. The mission's Curiosity rover isn't suited for detecting present-day life on the Red Planet, but it should give scientists a far better idea of what conditions on early Mars were like and whether life could have gained a foothold billions of years ago.

The real value of the Planetary Habitability Index developed by Schulze-Makuch and his colleagues would be to help scientists focus on potentially livable planets beyond our solar system. "Right now we have more than 700 exoplanets," he said. "In a few years, we'll have several thousand. You'll need to have something that you can use to prioritize. ... We have to have some way to assess what is the likelihood of life on them."

Schulze-Makuch acknowledged that the index as currently devised has lots of question marks attached to it. "One of the major points of the paper was that this classification system can always be updated, and it should be as more information becomes available," he said.

Where do you think we should focus our attention? On Titan? Mars? Ice-covered Europa? Ice-spewing Enceladus? Or on the hundreds of planets beyond the solar system? Feel free to weigh in with your comments below.